1. Field of the Invention
The present invention relates to a method and device for gamma correction in multichrome cathode ray tubes.
Its aim is more particularly to ensure conservation of the chromaticity of the colors displayed on the screen, over the whole dynamic range of adjustment of the contrast of the tube.
2. Description of the Prior Art
It is generally known that, in a conventional trichromatic cathode ray tube, the color picture points are each formed of three primary juxtaposed luminophore elements of red, green and blue color. These luminophore elements are excited by the flow of electrons from three electron guns assigned respectively to these three primary colors.
Such as seen by an observer, the color of this image point then consists in the additive synthesis of these three primary colors and depends on the relative proportions of the luminances L.sub.R, L.sub.G, L.sub.B of these elements.
Now, the luminances generated by the three guns have as expression: EQU L.sub.R =K.sub.R (M.sub.R).sup..gamma..sbsp.R (red gun) EQU L.sub.G =K.sub.G (M.sub.V).sup..gamma..sbsp.G (green gun) EQU L.sub.B =K.sub.B (M.sub.B).sup..gamma..sbsp.B (blue gun)
in which expressions, for a color, R, G, B shown in subscript:
the term M represents the cathode modulation;
the term .gamma. represents the gamma exponent (between 2 and 3);
the term K is a constant.
It proves then that the transfer function L=f(M) is different for each gun.
Furthermore, the contrast control acts simultaneously on the modulation of the three video channels, so that when this control varies the relative proportions of the luminances are modified because of the differences between the transfer functions of the guns. A variation of the chromaticity may then be observed depending on the contrast adjustment.
This drawback is particularly important in numerous applications which require considerable contrast variations and in which the colors form part of a symbology and have very precise meanings. Such is the case particularly in aeronautics.
For overcoming these disadvantages it has been proposed to introduce a correction (gamma correction) of type 1/.gamma..sub.i in each video channel so as to linearize the transfer curves L=f(M) of each of the guns.
However, this method comprises the following limitations:
the correction circuits placed directly in the video channels must have a wide pass band;
the high correction amplitude (M 1/.gamma.) requires use of expensive techniques for attaining the precision and stability required;
the luminance emitted by the tube is proportional to the contrast control; since perception of the eye is logarithmic, the result is a loss of sensitivity for the low levels; the video signals and the contrast control signal must therefore be compensated for accordingly;
in an assembly including a camera connected to the tube, the fact of linearizing the response of the tube impairs restitution of the grey levels from the camera whose transfer function (analysis gamma) is substantially the reverse of that of the uncorrected tube.